Most multi-cellular organisms are organized into tissues and organs which perform specialized functions. Thus, a system has evolved to transport materials between them. In higher animals, including mammals, this circulatory system is closed to improve the efficiency of transport. The flow of blood fluid through this closed cardiovascular system requires that the fluid be maintained under pressure and the regulation of the systemic arterial blood pressure requires a complex interaction of numerous factors including, e.g., fluid volume and vascular elasticity and caliber.
The maintenance of normal extracellular fluid volume depends primarily on the excretion of sodium (natriuresis) and water (diuresis) by the kidneys. This is determined by (1) the rate at which plasma is filtered at the glomerulus (glomerular filtration rate, or GFR) and (2) the degree to which sodium is actively reabsorbed along the renal tubule (with water following passively). The latter process is in part regulated by the adrenal steroid hormone aldosterone. It has been long believed that, in addition to GFR and aldosterone, there must be a "third factor" which also regulates sodium reabsorption. It is now apparent that many of the phenomena which required the postulation of a "third factor" can be explained by the effects of physical forces (e.g. blood pressure, red blood cell concentation and plasma viscosity) on sodium reabsorption. Nonetheless, the search continues for a "natriuretic hormone" which might directly inhibit tubular reabsorption.
There are several candidates for such a hormone, among which are included the natriuretic factor(s) recently isolated from atrial muscle cells. A natriuretic effect has been demonstrated by crude extracts of rat atrial tissue but not ventricular tissue. De Bold, A.J. et al., Life Sciences, 28:89-94 (1981), Garcia, R., Experientia, 38:1071-73 (1982), Currie, M.G. et al., Science 221:71-73 (1983). Various peptides with diuretic and natriuretic properties have been isolated from atrial tissue and sequenced. Flynn, T.G. et al., Biochem. Biophys. Res. Commun. 117:859-865 (1983), Currie, M.G. et al., Science 223:67-69 (1984), Kangawa, K. et al., Biochem. Biophys. Res. Commun. 118:131-139 (1984). The existence of these atrial natriuretic factors strengthens the long-held suspicion that the heart, aside from its obvious influence on renal perfusion, may play an important role in regulating renal sodium and water excretion. Stretching of the atria is known to induce diuresis and natriuresis, and this is possibly mediated by increased release of these factors.
A number of clinically important disease states are characterized by abnormal fluid volume retention. Congestive heart failure, cirrhosis of the liver and the nephrotic syndrome each lead to excessive fluid accumulation on the venous side of the circulation, the presumed common mechanism being under-perfusion of the kidneys leading to a fall in GFR. In addition the reduced renal perfusion stimulates excessive secretion of renin, a proteolytic enzyme whose action in the circulation leads to the formation of angiotensin. Angiotensin is a powerful constrictor of arterioles (which helps to maintain arterial pressure) and also stimulates release of the sodium-retaining hormone aldosterone by the adrenal gland (which further worsens fluid retention). These mechanisms do not, however, fully account for the fluid retention of the so-called "edematous states", and additional factors are likely to be involved. One important possibility is that a relative or absolute deficiency of atrial natriuretic factor, caused either by chronic over-stretching of the atrium (e.g., heart failure) or by inadequate stimulation of the atrium (e.g., cirrhosis and nephrotic syndrome), might contribute to the fluid retention.
An increase in extracellular fluid volume is also thought to contribute to the development of hypertension in many instances. Hypertension, or chronically elevated blood pressure, is one of the major causes of illness and death worldwide. It is estimated that more than 20 million Americans suffer from this disease whose complications include heart failure, heart attack, stroke and kidney failure. The major observed hemodynamic abnormality in chronic hypertension is increased resistance to the flow of blood through the arterioles. The mechanisms which lead to this increased "peripheral resistance" are, however, incompletely understood. In some cases inappropriate activity of the renin-angiotensin system or sympathetic nervous system may lead to excessive constriction of the arterioles; by "inappropriate" it is meant that the unknown signal(s) leading to this activity are not based upon a physiological need of the organism and thus lead to elevated blood pressure (whereas, in the example cited earlier, the increased renin secretion in the edematous states is a response to reduced arterial pressure and thus helps to restore or maintain normal pressure). In a substantial fraction of hypertensives however, inappropriate sodium and volume retention by the kidney is felt to either initiate or contribute to the elevated blood pressure. The responsible defect in kidney function and the mechanism whereby fluid retention leads to increased peripheral resistance are both unknown. It is certainly possible that deficiency of a natriuretic hormone could be responsible for these observations, particularly if the same substance also normally exerted a relaxant effect on arterioles.
Diuretic therapy is currently a mainstay in the treatment of hypertension, renal failure and the various edematous states (heart failure, etc.). Currently available pharmacological preparations have, however, several important limitations and undesirable effects. While their use may be directed at a specific abnormality (i.e. volume expansion), their multiple actions are undoubtedly not physiological, leading for instance to potassium depletion, increased retention of uric acid and abnormal glucose and lipid metabolism. In addition, all known diuretics profoundly stimulate the renin-angiotensin-aldosterone system, which counteracts their volume-depleting and blood pressure-lowering effects and leads to other unwanted effects. It would be desirable to provide a pharmacologically effective compound which can regulate blood pressure by providing a complete but controlled range of physiological responses.
Accordingly, it is the principal object of the present invention to provide methods and compounds for influencing fluid volume and blood pressure homeostasis in mammals.
It is another object of the present invention to provide methods and compounds which mimic the physiological regulation of fluid volume and blood pressure in mammals.